derives from holding up three fingers to form a W.
The W usually represents the slang term "Word" and sometimes "Whatever". "Threes" can also be used when someone says something stupid and you reply "What?" but instead, hold up three fingers and say, "Threes."

For example, "threes" can be used in situations like these:

Friend: "I'll meet you at the dining hall at 3, I have class now."
You: "Threes"

or

You: "She just cheated on me with that dude. But it's okay I'm move on and I'm gonna get with her best friend. Threes Bitch!"

and finally,

Friend: "My dog is black."
You: (must hold up three fingers in this case) "Threes to you making that dumbass remark."

To be called a three means you are stupid. that you dont understand concepts of which someone would be talking about and your pretty much the biggest dumb fuck the person calling you a three has ever met.

when asked what the word three means, dont tell the asker. theyre not supposed to know. the word could mean anything you would like it to mean, whether it be a sexy beast or flamming homosexual. but the person doesnt know it, so its descising your real thoughts.

calling them a three will make them look like a dumbfuck in two ways: pretending they know what it means, or getting extremely confused.